I don't know why this is necessarily incorrect. SG is just as acceptably an abbreviation for "SuicideGirls" as it is for "Suicide Girl."
For example, it's correct to say someone is an "SG photographer," which doesn't mean that one is a "Suicide Girl Photographer," it means that one is a "SuicideGirls Photographer"- ie, a photographer for SuicideGirls. Similarly, saying that one is an "SG girl" doesn't necessarily mean that one is a "Suicide Girl Girl," it could just as easily mean that one is a "SuicideGirls girl"- ie, a girl for SuicideGirls. This seems especially the case as SG is capitalized while "girl" isn't. So while "SuicideGirls girl" doesn't have a great ring to it, I don't think it's by any means incorrect.
People inevitably slam newish members for this, and I've always thought this was quibblingly picky at best, and outright wrong at worst.
Fatality
SUICIDEGIRL
USA
DEC 01, 2005 01:39 PM